A Matching Transformer is the solution when patching a laptop or iDevice into a XLR input on your mixer.
This device will "match" the two different input and output signals for optimum results.
It sums (combines) the stereo input channels to mono.
It attenuates the higher line or headphone output voltage level down to mic voltage level to avoid overloading and distorting the input.
It also balances the output, which allows for much longer cable runs without any audio degradation.